1. Field of the Invention
This invention relates to a cold-rolled high strength steel plate of a composite or multi-phase steel structure having a high r-value to ensure excellent deep drawability and baking hardenability, and to a method for producing such steel plates.
2. Description of the Prior Art
For the purpose of improving the gasoline mileage and safety of motor vehicles, attempts have been made for several years to use cold-rolled high strength steel sheet for the exterior and interior panels of motor vehicles. These attempts, however, mostly failed to reach the stage of application on an industrial scale due to problems such as clefts, wrinkles or other surface defects arising during forming, forming difficulty caused by large spring-back forces, or difficulty of spot welding. Recently, steel sheet with high formability such as Al-killed steel containing phosphorous, and the ferrite + martensite steel (the so-called "dual phase steel", "D.P. steel") have been used in some applications. With regard to D.P. steel, it has not yet been adapted for industrial applications in spite of its advantages, i.e., high strength, low yield ratio and baking hardenability (B.H.). However, D.P. steel has a serious problem in that the introduction of martensite or a low temperature transformation structure hereinafter referred to as second phase in ferrite causes a material drop in the value of r, an index of deep drawability, making it difficult to obtain an r-value higher than 1.5, that is, to achieve good deep drawability which is an important factor in formability. Furthermore, the strength of D.P. steel is too high to produce a steel in the strength class 35-40 kg/mm.sup.2 reproducibly.
Hence a need has continued to exist for a dual phase steel having a high r-value, in order to provide the steel with good deep drawing properties and the capability of being hardened by baking.